Mixing Displacement Procedural Texture
You can do this with newest Blender versions (as I remember from 2.78 and up).



First of all you'll need to turn on Experimental features (1) and select under Material tab Displacement as True (2). You can use Both but mixing Displacement with Bump gives weird results. Next step is to add Subdivision Surface modifier with Adaptive checked.

You can check whole tutorial about it here: Introduction to Microdisplacements